Method for preparation of lignin-based latex for binding and coating applications

The present invention relates to an ether derivative of lignin. In particular the present invention concerns novel lignin-based latexes, especially, lignin-nanoparticle-based latexes, their preparation method and their uses for binding and coating applications.

Paper and paperboard are renewable packaging materials but they do not as such have the required barrier properties to provide protection for example against moisture, oxygen, volatile aroma, grease and oil, as well as impact of light. At present, the required barrier properties are achieved especially by extrusion plastics (e.g. polyethylene), aluminium, and/or fluorochemicals, resulting in poor end-product recyclability and increasing safety concerns in food packaging materials. Typically, such barriers used are multilayer films.

Also latexes are commonly used as coating materials. Latexes (elastomers) are soft amorphous polymers, which are commonly formulated as water-based dispersions (latexes) and used in numerous products, e.g., converted paper, packaging materials, coatings, textiles and car tyres. However, the commercial products are mainly oil-based, such as styrene-butadiene, styrene-acrylate and polyvinyl-acetate copolymers.

Currently, fossil fuel-based polymer materials for coating applications put a great threaten to environment and human's health. Thus, there is a pressing need for alternative bio-based polymer materials to replace the source of oil-based materials.

One of the promising approaches is the development of bio-latex coating technology, such as designing natural rubber, starch, protein, hemicellulose, or cellulose functionalized latex dispersions. Vartiainen et al., for example, describe a bio-based multilayer barrier film produced by combining dispersion-coated cellulose nanofibrils, atomic layer deposited aluminium oxide, and extrusion-coated polyglycolic acid.

Also lignin has been researched to be used in rubber based materials to replace conventional rubber filler carbon black. However, it has been noted that lignin shows little or no reinforcing effect if directly mixed with rubber, presumably due to large particle size and lack of strong interfacial interactions between lignin and rubber matrix. Thus, lignin modification and hybrid fillers have been researched as an option. Jiang et al. have also used high temperature dynamic heat treatment in order to improve interaction between lignin and rubber. No optimal solution has been found.

Messmer et al. describe the effect of lignin treatment with tert-butyl hydroperoxide and sodium formaldehyde sulfoxylate on the properties of the latexes produced by emulsion copolymerization of styrene with n-butyl acrylate and methacrylic acid, with initiator introduced in a shot process.

One of the biggest challenges for bio-based barrier and binder coatings is simultaneously meet two contradictory requirements, the first being a low film formation temperature (less than 15° C.) that ensures the formation of smooth and flexible film at room temperature. The second being a sufficient film blocking resistance and film hardness, which are usually achieved by polymers with a glass transition temperature (T) higher than room temperature. Typically, the softer the polymer, the worse is the hardness and the higher is the blocking tendency, which restricts the performance and production efficiency of the barrier coating. Thus, the contradiction between these two properties typically results in compromised material properties.

Thus, there is a need to improve the functional properties of biopolymers for more demanding packaging applications. In particular, there is a clear need for new thermoplastic bio-based elastomers to replace the petroleum-derived synthetic polymers which are the main ones currently in use.

It is an aim of the present invention to reduce or even completely eliminate the above-mentioned problems encountered in the art.

This invention provides a lignin ether grafted with poly(alkyl acrylate). The lignin derivative is in particular obtained by introducing, via ether bonds, groups containing unsaturated bonds onto the lignin backbone, and by then reacting the derivative exhibiting unsaturated bonds with a reactant containing acrylate groups. By copolymerizing the modified lignin with a specific poly(alkyl acrylate), for example via emulsion polymerization, a lignin-based latex can be provided which is suitable for various end-use applications. Thus, it has been surprisingly found in the present invention that such lignin derivative provides improved barrier and binding properties despite of inherent heterogeneity of lignin.

The lignin-based latex finds uses in the paper, paperboard and packaging industry as well as broader coating and surface treatment products.

More specifically, the invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

According to a first aspect of the present invention, there is provided a lignin ether grafted with poly(alkyl acrylate).

According to a second aspect of the present invention, there is provided a latex comprising a lignin ether grafted with poly(alkyl acrylate).

According to a third aspect of the present invention, there is provided a method of manufacturing a biobased latex, comprising the steps of providing an allylated phenolic ether derivative of lignin; and grafting the allylated lignin ether derivative with a poly(alkyl acrylate) by emulsion polymerization.

According to a fourth aspect of the present invention, there is provided a biobased, water-borne latex formed by a method comprising the steps of providing an allylated phenolic ether derivative of lignin; and grafting the allylated lignin ether derivative with a poly(alkyl acrylate) by emulsion polymerization.

According to a fifth aspect of the present invention there is provided a biobased film formed by a water-borne latex comprising a lignin ether grafted with poly(alkyl acrylate) or a method comprising the steps of providing an allylated lignin ether derivative; and grafting the allylated lignin ether derivative with a poly(alkyl acrylate) by emulsion polymerization.

According to a sixth aspect of the present invention there is provided uses for latexes.

More specifically the present invention is characterized by what is stated in the independent claims.

Considerable advantages are obtained by the invention.

The present invention provides a sustainable and safe barrier coating comprised of lignin-nanoparticle-based latex dispersion, especially for fiber-based packaging materials. Use of lignin, the most abundant aromatic bioresource, to replace fossil-based chemicals reduces the environmental footprint of the packaging material thereof, and further improves recyclability of fiber-based packaging materials. Further, the non-toxicity of lignin in combination with its natural antioxidant activity, hydrophobicity and ultraviolet-shielding property provides a safer alternative for harmful aluminium or fluorochemicals in providing protection against oxygen, water vapour, oil, grease and light impact.

The lignin nanoparticles of spherical shape also enhances colloidal stability compared to bulk lignin and other biopolymers. In addition, lignin nanoparticles are functionalized with polymerization locus, which will co-polymerize with acrylic monomers and benefit the quality and runnability of the latex dispersion and further ensure the formation of a smooth and uniform layer.

The latex barrier coating technology of the present invention also enables the production of cost-effective products on-site with fewer production steps compared with applying extrusion plastics. Further, the method enables tailoring of the barrier properties to specific end-use requirements, and it also enables development of barrier coating formulations that adapt to all kinds of paper machines.

The present invention overcomes the challenges of the prior art with a unique latex morphology with soft-shell of acrylic polymer (with low T) and spherical hard-core lignin nanoparticles (with high T). Thus, the material of the present invention satisfy both the demand of a low film formation temperature and a sufficient flexibility of the barrier layer thus providing blocking resistance and required film hardness. Such is not achieved with a latex absent of a core-shell structure.

The allylated lignin ether derivative shows good binding properties, and films prepared from it have high flexibility with extensional strain up to 3000% and hydrophobicity with water contact angle up to 90°, as well as high transparency. In the packaging industry, high transparency is usually desirable to enable the visibility of coated products.

Further features and advantages of embodiments will become evident from the following description of preferred embodiments in which reference is made to the attached drawings.

In the present context, the term “biobased” refers to a material that comprises, consists or essentially consists of a substance (or substances) derived from living matter (biomass) and either occur naturally or are synthesized. In some embodiments, a part or all of the biobased material or biobased raw-materials are obtained from renewable sources, such as from biomass, including lignin.

As used herein, the term “average particle size” refers to the intensity weighted average hydrodynamic diameter.

As used herein, the term “about” refers to the actual given value, and also to an approximation to such given value that would reasonably be inferred to one of ordinary skill in the art, including approximations due to the experimental and/or measurement conditions for such given value.

Unless otherwise stated, properties that have been experimentally measured or determined herein have been measured or determined at room temperature.

Unless otherwise indicated, room temperature is 23° C.

Unless otherwise stated, properties that have been experimentally measured or determined herein have been measured or determined at atmospheric pressure.

In the present context, the expression “unsaturated groups” stands for groups which exhibit unsaturated bonds, such as double or triple bonds, either one or more per group—in case of several unsaturated bonds they may be conjugated or isolated—and which are capable of reacting with other groups, in particular with other groups of similar kind. In one embodiment, the groups have a double or triple bond.

According to an embodiment there is provided a lignin ether grafted with poly(alkyl acrylate).

According to one embodiment, the lignin ether is obtained by grafting poly(alkyl acrylate) to lignin ether derivative, in particular to allylated lignin ether derivative. In one particular embodiment, the lignin ether is obtained by grafting poly(alkyl acrylate) to allylic ether groups on the lignin.

According to an embodiment the biolatex is obtained by grafting poly(alkyl acrylate) to allylic ether groups on the lignin.

According to one embodiment, the poly(alkyl acrylate) is grafted to allylated lignin ether derivative obtained by reacting lignin exhibiting phenolic hydroxyl groups with a bi-functional reactant. In a preferred embodiment, the bi-functional reactant contains epoxy groups and unsaturated groups, in particular vinyl or allyl groups.

According to an embodiment the poly(alkyl acrylate) is grafted to ether groups on the lignin, in particular allylic ether groups, which are obtained by reacting the lignin exhibiting phenolic hydroxyl groups with a bi-functional reactant containing epoxy groups and unsaturated groups, in particular vinyl or allyl groups.

Thus, according to one embodiment, hydroxyl groups of lignin are functionalized by using especially allyl ether. Preferably, such functionalization is carried out by an anionic ring-opening reaction under alkaline conditions.

In the present context, the term “degree of substitution”, abbreviated “DS”, refers to the average amount of substituent groups (mmol) attached per gram of the derivative of lignin.

According to an embodiment the poly(alkyl acrylate) graft is obtained by polymerization, in particular by free radical polymerization, of an alkyl acrylate monomer or an alkyl (alk)acrylate monomer or a combination thereof, in the presence of the lignin ether. In particular alkyl-acrylate monomer is selected from the group of ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate and isobutyl acrylate and combinations thereof, whereas the alkyl (alk) acrylate monomer is preferably selected from the group of methyl methacrylate, n-hexyl methacrylate, n-octyl methacrylate and isooctyl acrylate monomers and combinations thereof. Preferably, the monomer is selected from the group of monomers, which yield homopolymer that have low glass transition temperature, for example their glass transition temperature is between −80° C. and 20° C., such as −60° C. to 10° C. or as −50° C. to 0° C., and which monomers have hydrophobic long alkyl groups. “Long alkyl groups” refer to alkyl groups having 4 to 30 carbon atoms, in particular 8 to 24 carbon atoms.

In other words, according to one embodiment a bi-functional monomer preferably containing reactive groups and unsaturated groups, is used for reacting with hydroxyl groups of lignin to introduce double bonds onto lignin molecular structure. Then the modified lignin is copolymerized with a poly(alkyl acrylate), for example via emulsion polymerization, such as the pre-emulsified semi-continuous emulsion polymerization method, to synthesize the lignin-based hybrid latex for the end-use applications.

According to one embodiment, the bi-functional reactant can be any reactant comprising reactive groups and unsaturated groups. In particular, the reactive groups are epoxy groups or alkyl halide groups, whereas the unsaturated groups are vinyl and/or allyl groups. The bi-functional reactant comprising epoxy groups and vinyl and/or allyl groups can be for example allyl glycidyl ether containing epoxy groups and allyl groups. The bi-functional reactant comprising alkyl halide groups and vinyl and/or allyl groups can be for example allyl bromide or allyl chloride or a mixture thereof.

According to one embodiment, the bi-functional reactant is selected from the group of allyl glycidyl ether, allyl bromide and allyl chloride and mixtures thereof.

Thus, in one embodiment, a bi-functional monomer, such as allyl glycidyl ether containing epoxy groups and vinyl/allyl groups, is introduced and chemically bonded to a lignin through etherification. Then the allylated lignin is grafted with an alkyl acrylate monomer to prepare lignin-nanoparticle-based latexes for coating applications for example using pre-emulsified semi-continuous emulsion polymerization.

For the present purpose, the term “pre-emulsified semi-continuous emulsion polymerization” stands for a continuously operated emulsion polymerization process, in which the monomers to be fed into the polymerization are pre-emulsified separately before they are feed in the polymerization reaction.

According to one embodiment, the allylic lignin ether is subjected to copolymerization with a combination of alkyl acrylate and alkyl (alk)acrylate monomers. According to one embodiment said combination comprises alkyl acrylate and alkyl (alk)acrylate monomers at a molar ratio of 20:80 to 80:20 or 30:70 to 70:30, such as 35:75 to 75:35, for example 35:75 to 40:60, for example 44:56.

The poly(alkyl acrylate), particularly n-butyl acrylate, can be obtained from fully biomass resources based on the bio-based alcohol fermented from glucose and the bio-based acrylic acid converted from lactic acid. By using biomass resources, the biomass content accounted for in the ready product is up to 70%, especially up to 62%.

The lignin can be any lignin. However, according to a preferred embodiment the lignin is selected from the group of unmodified lignin, alkali lignin, non-sulphurous lignin, Organosolv lignin, Kraft lignin, LignoBoost lignin or LignoForce lignin.